In order to produce a tread strip for automotive vehicle tyres, it is generally necessary for a number of differenct types of rubber mixtures to be combined. Thus, there is required a tread surface mixture which is made of a highly wear-resistant material, a shoulder strip mixture which is made of highly compressible material, a so-called "lower" rubber mixture which forms the bottom layer of the tread surface and may be softer than the tread surface, and the so-called "rubber pad" mixture to ensure good adhesion of the tread strip to the carcase of the tire.
These four mixtures have markedly different viscosities but must, nevertheless, be brought together in a single extrusion head to form a tread strip profile. This necessitates the provision of special measures in respect of the flow channels through which each mixture passes.
British Patent Specification No. 21 31 734B discloses an extrusion apparatus for producing flat profiles from two different types of mixtures. The extrusion head forming part of the extrusion apparatus is divided into two planes located one above the other. The head comprises a central portion which is fixedly connected to the extrusion devices, and upper and lower portions which are pivotable towards and away from the central portion. Flow channel inserts for each of the mixtures are provided in such an arrangement and are disposed, in the operative position of the head, between the central portion and one of the upper and lower portions.
It is necessary for the head portions to be pivotable so that the flow channel inserts of the head can periodically be changed or cleaned. Thus, the inserts may need to be changed if a different rubber or elastomeric mixture is to be used in one or all of the extrusion devices, or inserts with different flow channels and different pre-forming bar assemblies may also be required if the final profile is to be altered. The pivotability of the head portions means that the flow channels can be used in an optimum manner in the head.
In many instances of use, the profile strips need to be changed relatively frequently. For example, they need to be changed in order to produce tread strips for tyres of different sizes for cars or lorries or for producing special tyres for earth moving machines. Each change in the type of tread strip results necessitates the change of both the flow channel inserts and the pre-forming bar assembly.
After the pivotable head portions have been pivoted away from the central portion using hydraulic means, the flow channel inserts are extracted utilising hydraulic piston and cylinder arrangements. The insert members are secured to the piston rods for such arrangement by means described in U.S. Pat. No. 4,619,599.
The pre-forming bar assemblies, which are disposed at the outlet of the head, have, hitherto, been removed manually. They are then gripped by a crane or hoist and replaced by new bar assemblies defining different flow channels and profile bars.